Local and wide-area transmissions in a wireless broadcast network

ABSTRACT

To broadcast different types of transmission having different tiers of coverage in a wireless broadcast network, each base station processes data for a wide-area transmission in accordance with a first mode (or coding and modulation scheme) to generate data symbols for the wide-area transmission and processes data for a local transmission in accordance with a second mode to generate data symbols for the local transmission. The first and second modes are selected based on the desired coverage for wide-area and local transmissions, respectively. The base station also generates pilots and overhead information for local and wide-area transmissions. The data, pilots, and overhead information for local and wide-area transmissions are multiplexed onto their transmission spans, which may be different sets of frequency subbands, different time segments, or different groups of subbands in different time segments. More than two different types of transmission may also be multiplexed and broadcast.

The present Application for Patent is a continuation of patentapplication Ser. No. 10/968,787 entitled “Local And Wide-AreaTransmissions In A Wireless Broadcast Network” filed Oct. 18, 2004, nowU.S. Pat. No. 7,660,275, and Provisional Patent Application No.60/514,152, entitled, “Method for Transmitting Local and Wide-AreaContent Over a Wireless Multicast Network,” filed Oct. 24, 2009, nowexpired, both assigned to the assignee hereof and hereby expresslyincorporated by reference herein.

BACKGROUND

I. Field

The present invention relates generally to communication, and morespecifically to data transmission in a wireless communication network.

II. Background

Wireless and wireline broadcast networks are widely deployed to providevarious contents to a large group of users. A common wireline broadcastnetwork is a cable network that delivers multimedia content to a largenumber of households. A cable network typically includes headends anddistribution nodes. Each headend receives programs from various sources,generates a separate modulated signal for each program, multiplexes themodulated signals for all of the programs onto an output signal, andsends its output signal to the distribution nodes. Each program may bedistributed over a wide geographic area (e.g., an entire state) or asmaller geographic area (e.g., a city). Each distribution node covers aspecific area within the wide geographic area (e.g., a community). Eachdistribution node receives the output signals from the headends,multiplexes the modulated signals for the programs to be distributed inits coverage area onto different frequency channels, and sends itsoutput signal to households within its coverage area. The output signalfor each distribution node typically carries both national and localprograms, which are often sent on separate modulated signals that aremultiplexed onto the output signal.

A wireless broadcast network transmits data over the air to wirelessdevices within the coverage area of the network. A wireless broadcastnetwork is different from a wireline broadcast network in several keyregards. First, signals transmitted by different base stations in thewireless broadcast network interfere with one another if these signalsare not the same. In contrast, the output signal of each distributionnode is sent on dedicated cables and thus experiences no interferencefrom other distribution nodes. Second, each base station in the wirelessbroadcast network typically transmits a single radio frequency (RF)modulated signal that carries data for all programs being broadcast bythat base station. In contrast, each distribution node in the wirelinebroadcast network may multiplex individual modulated signals fordifferent programs onto different frequency channels. Because of thesedifferences, the techniques used to distribute programs in a wirelinebroadcast network are generally not applicable for a wireless broadcastnetwork.

There is therefore a need in the art for a wireless broadcast networkthat can efficiently broadcast different types of content with differentcoverage areas.

SUMMARY

Techniques for broadcasting different types of transmissions (e.g.,local and wide-area transmissions) in a wireless broadcast network aredescribed herein. As used herein, “broadcast” and “broadcasting” referto transmission of content/data to a group of users of any size and mayalso be referred to as “multicast” or some other terminology. Awide-area transmission is a transmission that may be broadcast by all ormany transmitters in the network. A local transmission is a transmissionthat may be broadcast by a subset of the transmitters for a givenwide-area transmission. Different local transmissions may be broadcastby different subsets of the transmitters for a given wide-areatransmission. Different wide-area transmissions may also be broadcast bydifferent groups of transmitters in the network. A venue transmissionmay also be broadcast by a smaller subset of a given subset oftransmitters for a given local transmission. The wide-area, local, andvenue transmissions may be viewed as different types of transmissionhaving different tiers of coverage, with the coverage area for eachtransmission being determined by all of the transmitters broadcastingthat transmission. The wide-area, local, and venue transmissionstypically carry different contents, but these transmissions may alsocarry the same content.

At each base station (or transmitter) in the wireless broadcast network,data for a wide-area transmission is processed in accordance with afirst coding and modulation scheme (or “mode”) selected for thewide-area transmission to generate data symbols for the wide-areatransmission. Data for a local transmission is processed in accordancewith a second coding and modulation scheme selected for the localtransmission to generate data symbols for the local transmission. Thefirst and second coding and modulation schemes may be selected based onthe desired coverage from the base station for the wide-area and localtransmissions, respectively. A time division multiplexed (TDM) pilotand/or a frequency division multiplexed (FDM) pilot used to recover thelocal and wide-area transmissions are generated. Overhead informationindicative of the time and/or frequency location of each data channelsent in the local and wide-area transmissions is also determined. Thedata channels carry multimedia content and/or other data being sent inthe local and wide-area transmissions.

The data, pilots, and overhead information for local and wide-areatransmissions may be multiplexed in various manners. For example, thedata symbols for the wide-area transmission may be multiplexed onto a“transmission span” allocated for the wide-area transmission, the datasymbols for the local transmission may be multiplexed onto atransmission span allocated for the local transmission, the TDM and/orFDM pilots for the wide-area transmission may be multiplexed onto atransmission span allocated for these pilots, and the TDM and/or FDMpilots for the local transmission may be multiplexed onto a transmissionspan allocated for these pilots. The overhead information for the localand wide-area transmissions may be multiplexed onto one or moredesignated transmission spans. The different transmission spans maycorrespond to (1) different sets of frequency subbands if FDM isutilized by the wireless broadcast network, (2) different time segmentsif TDM is utilized, or (3) different groups of subbands in differenttime segments if both TDM and FDM are utilized. Various multiplexingschemes are described below. More than two different types oftransmission with more than two different tiers of coverage may also beprocessed, multiplexed, and broadcast.

A wireless device in the wireless broadcast network performs thecomplementary processing to recover the data for the local and wide-areatransmissions. Various aspects and embodiments of the invention aredescribed in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout and wherein:

FIG. 1 shows a wireless broadcast network;

FIG. 2A shows coverage area for a wide-area transmission;

FIG. 2B shows coverage areas for different local transmissions;

FIG. 3A shows an FDM structure for broadcasting local and wide-areatransmissions;

FIG. 3B shows broadcast transmissions using the FDM structure in FIG.3A;

FIG. 4A shows a TDM structure for broadcasting local and wide-areatransmissions;

FIG. 4B shows broadcast transmissions using the TDM structure in FIG.4A;

FIG. 5 shows a super-frame structure for broadcasting local andwide-area transmissions;

FIG. 6 shows partitioning of data subbands into three disjoint sets;

FIG. 7 shows an FDM pilot for local and wide-area transmissions;

FIG. 8 shows a process for broadcasting local and wide-areatransmissions;

FIG. 9 shows a process for receiving local and wide-area transmissions;and

FIG. 10 shows a block diagram of a base station and a wireless device.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

FIG. 1 shows a wireless broadcast network 100 that can broadcastdifferent types of transmission such as, for example, wide-areatransmissions and local transmissions. Each wide-area transmission isbroadcast by a set of base stations in the network, which may includeall or many base stations in the network. Each wide-area transmission istypically broadcast over a large geographic area. Each localtransmission is broadcast by a subset of the base stations in a givenset for a given wide-area transmission. Each local transmission istypically broadcast over a smaller geographic area. For simplicity, thelarge geographic area for a wide-area transmission is also called a widecoverage area or simply a “wide area”, and the smaller geographic areafor a local transmission is also called a local coverage area or simplya “local area”. Network 100 may have a large coverage area such as theentire United States, a large region of the United States (e.g., thewestern states), an entire state, and so on. For example, a singlewide-area transmission may be broadcast over the entire state ofCalifornia, and different local transmissions may be broadcast overdifferent cities such as Los Angeles and San Diego.

For simplicity, FIG. 1 shows network 100 covering wide areas 110 a and110 b, with wide-area 110 a encompassing three local areas 120 a, 120 b,and 120 c. In general, network 100 may include any number of wide areaswith different wide-area transmissions and any number of local areaswith different local transmissions. Each local area may adjoin anotherlocal area or may be isolated. Network 100 may also broadcast any numberof different types of transmission designated for reception overgeographic areas of any number of different sizes. For example, network100 may also broadcast a venue transmission designated for receptionover a smaller geographic area, which may be portion of a given localarea. For simplicity, in much of the following description, network 100is assumed to cover a single wide area and multiple local areas for twodifferent transmission types.

FIG. 2A shows the coverage area for a wide-area transmission in network100. All base stations in a given wide area broadcast the same wide-areatransmission, and the network is referred to as a single frequencynetwork (SFN). If all base stations in the wide area broadcast the samewide-area transmission, then a wireless device may combine signalsreceived from different base stations for improved performance. At aphysical layer, the primary impairments to data reception in SFN arethermal noise and performance degradation due to time variation andexcess delay spread of the wireless channel. Delay spread is the timedifference between the earliest arriving signal instance (or multipath)and the latest arriving signal instance at the wireless device.

FIG. 2B shows the different coverage areas for different localtransmissions in network 100. The base stations in different local areastransmit different local transmissions, and the network is referred toas a multiple frequency network (MFN). The terms “SFN” and “MFN” arebroadcast terminology commonly used to describe the characteristics of anetwork, and MFN does not necessarily mean that different base stationstransmit on different radio frequencies. Even though the base stationsin different local areas broadcast different local transmissions, awireless device within the interior of a given local area may experiencelittle interference from the base stations in neighboring local areasbecause of the relatively large distance to the interfering basestations. For example, wireless device 1 in local area A, wirelessdevice 4 in local area B, and wireless device 6 in local area C mayexperience little interference from neighboring local areas. The localtransmission is essentially SFN in character for these interior wirelessdevices.

A wireless device near the boundary of a local area may observesignificant adjacent local channel interference (ALCI) from the signalstransmitted by base stations in neighboring local areas. For example,wireless device 2 in local area A may experience significant ALCI frombase stations in neighboring local areas B and C, wireless device 3 inlocal area B may experience significant ALCI from base stations inneighboring local areas A and C, and wireless device 5 in local area Cmay experience significant ALCI from base stations in neighboring localareas A and B. The network is essentially MFN in character for theseperipheral wireless devices. The ALCI results in additional performancedegradation over the SFN case. If data is processed and transmitted inthe same manner for both SFN and MFN, then the ALCI observed by theperipheral wireless devices in the MFN case degrades the received signalquality at these wireless devices and causes a reduction in coverage atthe boundary of neighboring local areas.

In general, the coverage for each type of transmission (e.g., wide-areaor local) may be matched to the usage requirement for that transmissiontype. A transmission with wider applicability may be sent to by wirelessdevices in a larger geographic area. Conversely, a transmission withmore limited applicability may be sent to wireless devices in a smallergeographic area.

Network 100 may be designed to provide good performance for both localand wide-area transmissions. This may be achieved by performing thefollowing:

-   -   Multiplexing the local and wide-area transmissions in time,        frequency, and/or code domain so that interference between the        two types of transmission is reduced;    -   Transmitting the local and wide-area transmissions (as well as        their associated pilots) based on different characteristics of        MFN and SFN, respectively; and    -   Providing flexibility in resource allocation to meet variable        (source) rate demands of the local and wide-area transmissions.        The local transmissions are sent based on MFN characteristics to        provide better coverage for wireless devices located at the        edges of the local areas. Wide-area transmissions for different        wide areas are also MFN in character at the boundary between        these wide areas and may also be sent using the techniques        described herein. Each of the above three aspects is described        in detail below.

1. Multiplexing Local and Wide-Area Transmissions

FIG. 3A shows an FDM structure 300 that may be used to broadcast localand wide-area transmissions over a given system bandwidth in amulti-carrier network. FDM structure 300 supports reception of bothlocal and wide-area transmissions by a receiver tuned to a single radiofrequency, and is different from a scheme that sends local and wide-areatransmissions using different radio frequencies. The overall systembandwidth is divided into multiple (N) orthogonal frequency subbands byusing a multi-carrier modulation technique such as orthogonal frequencydivision multiplexing (OFDM) or by some other construct. These subbandsare also called tones, carriers, subcarriers, bins, and frequencychannels. With OFDM, each subband is associated with a respectivesubcarrier that may be modulated with data. Of the N total subbands, Usubbands may be used for data and pilot transmission and are called“usable” subbands, where U≦N. The G remaining subbands are not used andare called “guard” subbands, where N=U+G. As a specific example, thenetwork may utilize an OFDM structure with N=4096 total subbands, U=4000usable subbands, and G=96 guard subbands. In general, N, U, and G may beany values. For simplicity, the following description assumes that all Nsubbands are usable for transmission, i.e., U=N and G=0 so that thereare no guard subbands.

In each symbol period with data transmission, P subbands out of the Nusable subbands may be used for an FDM pilot and are called “pilot”subbands, where P<N. A pilot is typically composed of known modulationsymbols that are processed and transmitted in a known manner. Theremaining D usable subbands may be used for data transmission and arecalled “data” subbands, where D=N−P. A TDM pilot may also be transmittedin some symbol periods on all N usable subbands.

For the embodiment shown in FIG. 3A, an FDM pilot is transmitted on Ppilot subbands that are distributed across the entire system bandwidthto provide better sampling of the frequency spectrum. The D datasubbands may be allocated to local transmission, wide-area transmission,overhead information, and so on. A set of L_(sb) subbands may beallocated for local transmission, and a set of W_(sb) subbands may beallocated for wide-area transmission, where W_(sb)+L_(sb)≦D. The W_(sb)subbands for wide-area transmission and the L_(sb) subbands for localtransmission may be distributed across the entire system bandwidth toimprove frequency diversity, as shown in FIG. 3A. The W_(sb) subbandscarry data for wide-area transmission (or simply, wide-area data), andthe L_(sb) subbands carry data for local transmission (or simply, localdata).

FIG. 3B shows data transmission for different local areas using FDMstructure 300. To minimize interference between the local and wide-areatransmissions, all base stations in a given wide area may use the sameset of W_(sb) subbands to broadcast the wide-area transmission. Basestations in different local areas may broadcast different localtransmissions on the set of L_(sb) subbands allocated for the localtransmissions. The number of subbands allocated for the local andwide-area transmissions may be varied based on resource requirements.For example, W_(sb) and L_(sb) may be varied (1) dynamically from symbolto symbol or from time slot to time slot, (2) based on time of the day,day of the week, and so on, (3) based on a predetermined schedule, or(4) based on any combination of the above. For example, W_(sb) andL_(sb) may be dynamically varied during a portion of each weekday, fixedduring the remaining portion of each weekday, and set based on apredetermined schedule on the weekend.

To simplify resource allocation and improve frequency diversity, the Nusable subbands may be arranged into M “interlaces” or disjoint subbandsets. The M interlaces are disjoint in that each of the N usablesubbands belongs to only one interlace. Each interlace contains P usablesubbands, where N=M·P. The P subbands in each interlace may be uniformlydistributed across the N usable subbands such that consecutive subbandsin each interlace are spaced apart by M subbands. For the exemplary OFDMstructure described above, M=8 interlaces may be formed, with eachinterlace containing P=512 usable subbands that are uniformly spacedapart by 8 subbands. The P usable subbands in each interlace are thusinterlaced with the P usable subbands in each of the other M−1interlaces.

An exemplary OFDM structure and interlace scheme have been describedabove. Other OFDM structures and subband allocation schemes may also beused to support FDM of local and wide-area transmissions.

FIG. 4A shows a TDM structure 400 that may also be used to broadcastlocal and wide-area transmissions in a single-carrier or multi-carriernetwork. The transmission timeline is divided into frames 410, with eachframe having a predetermined time duration. The frame duration may beselected based on various factors such as, e.g., the amount of timediversity desired for data transmission. Each frame includes a field 412carrying pilot and overhead information, a segment 414 carryingwide-area data, and a segment 416 carrying local data. Each frame mayalso include other fields for other information.

FIG. 4B shows data transmissions for different local areas using TDMstructure 400. To minimize interference between the local and wide-areatransmissions, wide-area segment 414 for all base stations in a givenwide area may be time-aligned so that these base stations broadcast thewide-area transmission at the same time. The base stations in differentlocal areas may broadcast different local transmissions in segment 416.The sizes of segments 414 and 416 may be varied dynamically or in apredetermined manner based on resource requirements.

For FDM structure 300 in FIG. 3A and TDM structure 400 in FIG. 4A, thelocal and wide-area transmissions are multiplexed in frequency and time,respectively, such that the two types of transmission minimally overlapone another. This alignment avoids or minimizes interference between thetwo transmission types. However, strict adherence to the non-overlappingof different transmission types is not necessary. Furthermore, differentlocal areas may have different frequency or time allocations. Ingeneral, various multiplexing structures may be used to broadcastdifferent types of transmission with different coverage areas. Aspecific multiplexing structure suitable for an OFDM-based wirelessbroadcast network is described below.

FIG. 5 shows an exemplary super-frame structure 500 that may be used tobroadcast local and wide-area transmissions in an OFDM-based wirelessbroadcast network. Data transmission occurs in units of super-frames510. Each super-frame spans a predetermined time duration, which may beselected based on various factors such as, for example, the desiredstatistical multiplexing for data streams being broadcast, the amount oftime diversity desired for the data streams, acquisition time for thedata streams, buffer requirements for the wireless devices, and so on. Asuper-frame size of approximately one second may provide a good tradeoffbetween the various factors noted above. However, other super-framesizes may also be used.

For the embodiment shown in FIG. 5, each super-frame 510 includes aheader segment 520, four equal-size frames 530 a through 530 d, and atrailer segment 540, which are not shown to scale in FIG. 5. Table 1lists the various fields for segments 520 and 540 and for each frame530.

TABLE 1 Fields Description TDM pilot TDM pilot used for signaldetection, frame synchronization, frequency error estimation, and timesynchronization. Transition Pilot used for channel estimation andpossibly time pilot synchronization and sent at the boundary ofwide-area and local fields/transmissions. WIC Wide-area identificationchannel (WIC) - carry an identifier assigned to the wide area beingserved. LIC Local identification channel (WIC) - carry an identifierassignedto the local area being served. Wide-Area Wide-area overheadinformation symbol (OIS) - carry OIS overhead information (e.g.,frequency/time location and allocation) for each data channel being sentin the Wide-Area Data field. Local OIS Local OIS - carry overheadinformation for each data channel being sent in the Local Data field.Wide-Area Carry data channels for wide-area transmission. Data LocalData Carry data channels for local transmission.

For the embodiment shown in FIG. 5, different pilots are used fordifferent purposes. A TDM pilot is transmitted at or near the start ofeach super-frame and may be used for the purposes noted in Table 1. Atransition pilot is sent at the boundary between local and wide-areafields/transmissions, allows for seamless transition between the localand wide-area fields/transmissions, and may be generated as describedbelow.

The local and wide-area transmissions may be for multimedia content suchas video, audio, teletext, data, video/audio clips, and so on, and maybe sent in separate data streams. For example, a single multimedia(e.g., television) program may be sent in three separate data streamsfor video, audio, and data. The data streams are sent on data channels.Each data channel may carry one or multiple data streams. A data channelcarrying data streams for a local transmission is also called a “localchannel”, and a data channel carrying data streams for a wide-areatransmission is also called a “wide-area channel”. The local channelsare sent in the Local Data fields and the wide-area channels are sent inthe Wide-Area Data fields of the super-frame.

Each data channel may be “allocated” a fixed or variable number ofinterlaces in each super-frame depending on the payload for the datachannel, the availability of interlaces in the super-frame, and possiblyother factors. Each data channel may be active or inactive in any givensuper-frame. Each active data channel is allocated at least oneinterlace. Each active data channel is also “assigned” specificinterlaces within the super-frame based on an assignment scheme thatattempts to (1) pack all of the active data channels as efficiently aspossible, (2) reduce the transmission time for each data channel, (3)provide adequate time-diversity for each data channel, and (4) minimizethe amount of signaling needed to indicate the interlaces assigned toeach data channel. For each active data channel, the same interlaceassignment may be used for the four frames of the super-frame.

The Local OIS field indicates the time-frequency assignment for eachactive local channel for the current super-frame. The Wide-Area OISfield indicates the time-frequency assignment for each active wide-areachannel for the current super-frame. The Local OIS and Wide-Area OIS aresent at the start of each super-frame to allow the wireless devices todetermine the time-frequency location of each data channel of interestin the super-frame.

The various fields of the super-frame may be sent in the order shown inFIG. 5 or in some other order. In general, it is desirable to send theTDM pilot and overhead information early in the super-frame so that theTDM pilot and overhead information can be used to receive the data beingsent later in the super-frame. The wide-area transmission may be sentprior to the local transmission, as shown in FIGS. 4A and 5, or afterthe local transmission.

FIG. 5 shows a specific super-frame structure. In general, a super-framemay span any time duration and may include any number and any type ofsegments, frames, and fields. However, there is normally a useful rangeof super-frame durations related to acquisition time and cycling timefor the receiver electronics. Other super-frame and frame structures mayalso be used for broadcasting different types of transmission, and thisis within the scope of the invention.

Time division multiplexing of the local and wide-area transmissions, asshown in FIG. 5, allows the wide-area transmission to enjoy theadvantages of OFDM within an SFN context, without interference fromlocal transmissions. Since only local or wide-area transmission is sentat any given moment with TDM, the local and wide-area transmissions maybe broadcast using different transmission parameters that may beindependently optimized to achieve good performance for the local andwide-area transmissions, respectively, as described below.

2. Data Transmission

The wide-area channels that are broadcast in each super-frame may bepacked as efficiently as possible. All base stations in a given widearea broadcast the same wide-area transmission in the four Wide-AreaData fields of each super-frame. A wireless device may then combinewide-area transmissions received from any number of base stations toimprove data reception performance.

The base stations in different local areas broadcast different localtransmissions in the four Local Data fields of each super-frame. Aperipheral wireless device located near the boundary of neighboringlocal areas would then observe adjacent local channel interference(ALCI), which degrades the received signal quality at the device.Received signal quality may be quantified by asignal-to-noise-and-interference ratio (SINR) or some other measure. Theperipheral wireless device would achieve a lower SINR because of thedegradation due to ALCI. At a base station, data for the localtransmission is processed with a coding and modulation scheme thatrequires a particular SINR for proper reception. ALCI has the effect ofshrinking the local area since a given wireless device can achieve therequired SINR in a smaller area in the presence of ALCI.

Various techniques may be used to improve coverage for the localtransmission. These techniques typically trade off performance in theinterior of the local area in order to extend coverage at the boundary.These techniques include partial loading and coding/modulationselection.

With partial loading, which is also called frequency reuse, not allsubbands usable for data transmission are actually used to transmitdata. Furthermore, neighboring local areas may be assigned subbands suchthat their local transmissions interfere as little as possible with oneanother. This may be achieved with orthogonal partial loading or randompartial loading.

With orthogonal partial loading, neighboring local areas are assigneddisjoint or non-overlapping sets of subbands. The base stations in eachlocal area then broadcast the local transmission on the set of subbandsassigned to that local area. Since the subband sets are disjoint, thewireless devices in each local area observe no ALCI from base stationsin neighboring local areas.

FIG. 6 shows an exemplary partitioning of the D data subbands into threedisjoint sets labeled as S₁, S₂ and S₃. In general, each set may containany number of data subbands and any one of the D data subbands. Thesubbands for each set may also change dynamically or in a predeterminedmanner. To achieve frequency diversity, each set may contain subbandstaken from across the D data subbands. The subbands in each set may beuniformly or non-uniformly distributed across the D data subbands.

Referring back to FIG. 2B, local area A may be assigned subband set S₁,local area B may be assigned subband set S₂, and local area C may beassigned subband set S₃. The base stations in local area A thenbroadcast the local transmission for local area A on subband set S₁, thebase stations in local area B broadcast the local transmission for localarea B on subband set S₂, and the base stations in local area Cbroadcast the local transmission for local area C on subband set S₃.

FIGS. 2B and 6 show a case with three local areas. Orthogonal partialloading may be extended to any number of local areas. Q disjoint subbandsets may be formed for Q neighboring local areas, where Q>1. The Q setsmay contain the same or different numbers of subbands. For the interlacescheme described above, the M−1 interlaces available for datatransmission may be allocated to the Q sets. Each set may contain anynumber of interlaces. The interlaces for each set may change dynamicallyor in a predetermined manner. Each local area is assigned a respectiveset of interlaces for local transmission. Frequency planning may beperformed across the entire network to ensure that neighboring localareas are assigned disjoint sets.

With random partial loading, each local area is allocated K datasubbands, where K≦D, and the base stations in that local area broadcaststhe local transmission on K subbands selected in a pseudo-random mannerfrom among the D data subbands. For each local area, a pseudo-randomnumber (PN) generator may be used to select a different set of Ksubbands in each symbol period. Different local areas may use differentPN generators so that the subbands used by each local area arepseudo-random with respect to the subbands used by neighboring localareas. In effect, the local transmission for each local area hops acrossthe D data subbands. ALCI is observed whenever a collision occurs andneighboring local areas use the same subband in the same symbol period.However, ALCI is randomized due to the pseudo-random manner in which theK subbands are selected in each symbol period for each local area. Awireless device has knowledge of the hopping performed by the basestations and can perform the complementary de-hopping to recover thelocal transmission.

For random partial loading, the probability of collision decreases andthe amount of ALCI decreases as K decreases. Coverage may thus beextended with a smaller value of K. However, a smaller K also results inlower overall throughput for a given coding and modulation scheme. K maythus be selected based on a tradeoff between coverage area and overallthroughput.

For partial loading of any type, the transmit power for each subbandused for data transmission may be increased without increasing the totaltransmit power. The total transmit power may be distributed over the Ksubbands used for local transmission in each symbol period, which may becalled the “active” subbands. If K subbands are used for localtransmission and D subbands are used for wide-area transmission, whereK<D with partial loading, then the transmit power per active subband ishigher for the local transmission than for the wide-area transmission.The received signal quality per active subband is thus higher withpartial loading, which improves the signal-to-noise ratio for thesubband at a receiver.

Orthogonal and random partial loading may be performed for only datasubbands, only pilot subbands, or both data and pilot subbands.Orthogonal and random partial loading can improve coverage at theexpense of a lower overall throughput. This is because fewer subbandsare used for data transmission with partial loading, and fewerinformation bits may be sent in each symbol period on these fewersubbands. The number of subbands to use for local transmission may beselected based on a tradeoff between improved coverage and overallthroughput.

The network may support a set of transmission modes, or simply “modes”.Each mode is associated with a particular coding scheme or code rate, aparticular modulation scheme, a particular spectral efficiency, and aparticular minimum SINR required to achieve a specified level ofperformance, e.g., 1% packet error rate (PER) for a non-fading AWGNchannel. Spectral efficiency may be given in units of information bitsper modulation symbol and is determined based on the code rate and themodulation scheme. In general, modes with lower spectral efficiencieshave lower required SINRs. For each mode, the required SINR may beobtained based on the specific system design (such as the code rate,interleaving scheme, and modulation scheme used for that mode) and for aparticular channel profile. The required SINR may be determined bycomputer simulation, empirical measurements, and so on.

The coverage area for a local transmission may be adjusted by selectingan appropriate mode to use for the local transmission. A mode with alower required SINR may be used for the local transmission in order toextend coverage near the boundary of neighboring local areas. Theparticular mode to use for the local transmission may be selected basedon a tradeoff between improved coverage and spectral efficiency. Thecoverage for a wide-area transmission may similarly be adjusted byselecting an appropriate mode to use for the wide-area transmission. Ingeneral, the same or different modes may be used for local and wide-areatransmissions.

The coverage for the local transmission may be improved with partialloading and/or mode selection. Coverage may be extended by using asmaller percentage of the usable subbands and/or by selecting a modewith a lower spectral efficiency. An information bit rate (R) may beexpressed as: R=η×K, where η is the spectral efficiency for the selectedmode and K is the number of active subbands. A given information bitrate may be achieved by using (1) a subset of all data subbands and amode with a higher spectral efficiency or (2) all data subbands and amode with a lower spectral efficiency. It can be shown that option (2)can provide better performance (e.g., wider coverage for a given PER)than option (1) for certain operating scenarios (e.g., for randompartial loading and without interference estimation).

3. Pilot Transmission

FIG. 7 shows a pilot transmission scheme that can support both local andwide-area transmissions. For simplicity, FIG. 7 shows pilot transmissionfor one frame of a super-frame. Each base station transmits thetransition pilot between local and wide-area fields/transmissions. Eachbase station also transmits the FDM pilot on one interlace in eachsymbol period with data transmission. For the embodiment shown in FIG.7, eight interlaces are available in each symbol period, and the FDMpilot is transmitted on interlace 3 in even-numbered symbol periodindices and on interlace 7 in odd-numbered symbol period indices, whichmay be denoted as a {3, 7} staggering pattern. The FDM pilot may also betransmitted with other staggering patterns such as, e.g., the {1, 2, 3,4, 5, 6, 7, 8} and {1, 4, 7, 2, 5, 8, 3, 6} patterns.

As shown in FIG. 7, the FDM pilot is transmitted during the wide-areatransmission as well as during the local transmission. The FDM pilot maybe used to derive (1) a channel estimate for the wide-area transmission,which is also called a wide-area channel estimate, and (2) a channelestimate for the local transmission, which is also called a localchannel estimate. The local and wide-area channel estimates may be usedfor data detection and decoding for the local and wide-areatransmissions, respectively.

The FDM pilot transmitted during the wide-area transmission is called awide-area FDM pilot and may be designed to facilitate wide-area channelestimation. The same wide-area FDM pilot may be transmitted across theentire wide area. The FDM pilot transmitted during the localtransmission is called a local FDM pilot and may be designed tofacilitate local channel estimation. Different local FDM pilots may betransmitted for different local areas to allow the wireless devices toobtain local channel estimates for the different local areas. Thedifferent local FDM pilots interfere with one another at the boundary ofneighboring local areas, similar to the ALCI for the different localtransmissions. The local FDM pilots may be designed such that a goodlocal channel estimate may be derived in the presence of pilotinterference from neighboring local areas. This may be achieved byorthogonalizing or randomizing the local FDM pilots for different localareas in frequency, time, and/or code domain, as described below.

FIG. 7 also shows an embodiment of the local FDM pilot. A set of Pmodulation symbols is used for the P pilot subbands for the local FDMpilot. The P modulation symbols may be multiplied with a first sequenceof complex values across frequency and/or a second sequence of complexvalues across time to generate the pilot symbols for the local FDMpilot. The first sequence is denoted as {S(k)}, where S(k) is thecomplex value for subband k. The second sequence is denoted as {C(n)},where C(n) is the complex value for symbol period n. Differentcharacteristics may be obtained for the local FDM pilot by usingdifferent types of first and second sequences.

A PN generator may be used to generate the first sequence of complexvalues. The PN generator may be a linear feedback shift register (LFSR)that implements a selected generator polynomial, e.g., g(x)=x¹⁵+x¹⁴+1.The PN generator is initialized to a particular seed value (or initialstate) at the start of each symbol period and generates a sequence ofpseudo-random bits. These bits are used to form the complex values forthe first sequence.

The pilot symbols for the local FDM pilot for a given local area may beexpressed as:P(k,n)=S(k)·C(n)  Eq (1)where P(k,n) is the pilot symbol for subband k in symbol period n.Equation (1) assumes that the modulation symbols used for the local FDMpilot have values of 1+j0.

The received pilot symbols at a wireless device may be expressed as:Y(k,n)=H(k,n)·P(k,n)+H _(I)(k,n)·P _(I)(k,n)+w(k,n),  Eq (2)where P(k,n) is a pilot symbol sent on subband k in symbol period n by abase station in a desired local area (i.e., the desired base station);

-   -   H(k,n) is an actual channel response for the desired base        station;    -   P_(I)(k,n) is a pilot symbol sent on subband k in symbol period        n by an interfering base station in a neighboring local area;    -   H_(I)(k,n) is an actual channel response for the interfering        base station;    -   Y(k,n) is a received pilot symbol for subband k in symbol period        n; and    -   w(k,n) is noise for subband k in symbol period n.        For simplicity, equation (2) assumes the presence of one desired        base station and one interfering base station, which is denoted        by the subscript I.

The local FDM pilots for different local areas may be orthogonalized intime and/or frequency by transmitting these local FDM pilots indifferent symbol periods and/or subbands, respectively. However, fewerpilot symbols would be sent for the local FDM pilot in each local area,and thus fewer pilot symbols would be available for local channelestimation.

The local FDM pilots for different local areas may also beorthogonalized and/or randomized in the code domain by using differentorthogonal and/or pseudo-random sequences, respectively, for these localFDM pilots. Various code orthogonalization/randomization techniques maybe used for the local FDM pilots, including orthogonal scrambling,random scrambling, and orthogonal and random scrambling.

For orthogonal scrambling, the local FDM pilots for different localareas are multiplied with orthogonal sequences across symbol periods.The pilot symbols for the desired and interfering local areas may thenbe expressed as:P(k,n)=S(k)·C(n) and P _(I)(k,n)=S(k)·C _(I)(n),  Eq (3)where {C(n)} is orthogonal to {C_(I)(n)}. As shown in equation (3), thesame PN sequence is used to generate the first sequence of complexvalues {S(k)} for both the desired and interfering local areas. However,different orthogonal sequences {C(n)} and {C_(I)(n)} are used for thedesired and interfering local areas.

A wireless device may derive a local channel estimate by first obtaininga complex channel gain estimate for each pilot subband used for thelocal FDM pilot, as follows:Ĥ _(p)(k)=P(k,n)/S(k)  Eq (4)Equation (4) removes the effects of the PN sequence across the pilotsubbands, which is also called descrambling. The wireless device obtainsP channel gain estimates for P uniformly distributed pilot subbands. Thewireless device next performs a P-point inverse discrete Fouriertransform (IDFT) on the P channel gain estimates to obtain a P-tapleast-squares impulse response estimate, which may be expressed as:ĥ _(os)(l,n)=h(l)·C(n)+h _(I)(l)·C _(I)(n)+w(l,n),  Eq (5)where l is an index for the P′ channel taps of the impulse responseestimate;

-   -   h(l) is the actual impulse response for the desired base        station;    -   h_(I)(l) is the actual impulse response for the interfering base        station;    -   ĥ_(os)(l,n) is the least-squares impulse response estimate for        symbol period n, where the subscript “os” denotes orthogonal        scrambling; and    -   w(l,n) is the noise in symbol period n.        Equation (5) assumes that the actual channel impulse response        for each base station is constant over the time duration of        interest, so that h(l) and h_(I)(l) are not functions of symbol        period n.

An impulse response estimate {tilde over (h)}_(os)(l) for the desiredlocal area may then be obtained by filtering the least-squares impulseresponse estimates for different symbol periods, as follows:

$\begin{matrix}\begin{matrix}{{{{\overset{\sim}{h}}_{os}(l)} = {\frac{1}{L} \cdot {\sum\limits_{n = {{- {({L - 1})}}/2}}^{{({L - 1})}/2}{{{\hat{h}}_{os}\left( {l,n} \right)} \cdot {C^{*}(n)}}}}},} \\{= {{\frac{1}{L} \cdot {\sum\limits_{n = {{- {({L - 1})}}/2}}^{{({L - 1})}/2}{{h(l)} \cdot {C(n)} \cdot {C^{*}(n)}}}} + {{h_{I}(l)} \cdot {C_{I}(n)} \cdot}}} \\{{{C^{*}(n)} + {{w\left( {l,n} \right)} \cdot {C^{*}(n)}}},} \\{{= {{h(l)} + {\overset{\sim}{w}\left( {l,n} \right)}}},}\end{matrix} & {{Eq}\mspace{20mu}(6)} \\{{where}{{\sum\limits_{n = {{- {({L - 1})}}/2}}^{{({L - 1})}/2}{{C_{I}(n)} \cdot {C^{*}(n)}}} = 0}} & \;\end{matrix}$since C(n) and C_(I)(n) are orthogonal sequences;

-   -   w(l,n) is the post-processed noise; and    -   L is the length of the orthogonal sequences (e.g., L=3).        The index of summation in equation (6) is for an odd value of L        and is different for an even value of L. A wireless device        located within the interfering local area may derive an impulse        response estimate {tilde over (h)}_(os,I)(l) for that local area        by multiplying {tilde over (h)}_(os)(l,n) with C_(I) ^(*)(n) and        integrating over the length of the orthogonal sequence. As shown        in equation (6), orthogonal scrambling can cancel pilot        interference from the neighboring local area. However, this        orthogonality may be disturbed due to channel time variations.

The orthogonal sequences may be defined in various manners. In oneembodiment, the orthogonal sequences are defined as follows:C(n)=1 and C _(I)(n)=e ^(j2π·n/L), for n=0 . . . (L−1).  Eq(7)

For random scrambling, the pilot symbols for the desired local area arepseudo-random with respect to the pilot symbols for the interferinglocal area. The pilot symbols may be considered to be independently andidentically distributed (i.i.d.) across time, frequency, and localareas. Pseudo-random pilot symbols may be obtained by initializing thePN generators for different local areas with different seed values thatare dependent on symbol period n and the local area identifier.

For random scrambling, a least-squares impulse response estimateĥ_(rs)(l) may be obtained by performing (1) descrambling as shown inequation (4) to remove the PN sequence for the desired local area, (2)post processing to obtain P channel gain estimates, and (3) an IDFT onthe P channel gain estimates, as described above. The least-squaresimpulse response estimate may be expressed as:ĥ _(rs)(l)=h(l)+g _(I)(l,n)+w(l,n),  Eq (8)where g_(I)(l,n) is the interference to the l-th tap of ĥ_(rs)(l) andthe subscript “rs” denotes random scrambling. The interferenceg_(I)(l,n) results from the channel impulse response h_(I)(l) for theinterfering local area being smeared across the P taps of ĥ_(rs)(l) bythe PN sequences for the local and interfering local areas. Theleast-squares impulse response estimate may be used directly as theimpulse response estimate for the desired local area. Equation (8)indicates that random scrambling only smears out (and does not suppressor cancel) the pilot interference from the neighboring local area.Thresholding may be performed to retain channel taps that exceed apredetermined threshold and to zero out channel taps below thepredetermined threshold. The thresholding can remove much of the pilotinterference and may provide performance that is comparable to thatachieved with orthogonal scrambling. In addition, with randomscrambling, channel estimation performance is not dependent onorthogonality and may be more robust in certain operating environments.

For orthogonal and random scrambling, the local FDM pilots for differentlocal areas are multiplied with different PN sequences across subbandsand further multiplied with different orthogonal sequences across symbolperiods. The pilot symbols for the desired and interfering local areasmay be expressed as:P(k,n)=S(k)·C(n) and P _(I)(k,n)=S _(I)(k)·C _(I)(n),  Eq(9)where {S(k)} and {S_(I)(k)} are different pseudo-random sequences, and{C(n)} and {C_(I)(n)} are different orthogonal sequences.

For orthogonal and random scrambling, a least-squares impulse responseestimate ĥ_(or)(l,n) may be obtained by performing the processingdescribed above for orthogonal scrambling. The least-squares impulseresponse estimate may expressed as:ĥ _(or)(l,n)=h(l)·C(n)+g _(I)(l)·C _(I)(n)+w(l,n),  Eq (10)where the subscript “or” denotes orthogonal and random scrambling. Animpulse response estimate {tilde over (h)}_(or)(l) for the desired localarea may be obtained by multiplying ĥ_(or)(l,n) with C^(*)(n) andintegrating over the length of the orthogonal sequence, as shown inequation (6).

The sampled channel impulse response for each (local or wide) areacontains up to N taps, where N=M·P. The channel impulse response may beviewed as being composed of a main channel and an excess channel. Themain channel contains the first P taps of the channel impulse response.The excess channel contains the remaining N−P taps. If the FDM pilot istransmitted on one interlace with P subbands, then an impulse responseestimate {tilde over (h)}_(os)(l), ĥ_(rs)(l), or {tilde over(h)}_(or)(l) with P taps may be obtained based on the received FDMpilot. In general, the length of the impulse response estimate isdetermined by the number of different subbands used for the FDM pilot. Alonger channel impulse response estimate with more than P taps may beobtained by transmitting the FDM pilot on more interlaces. For example,the FDM pilot may be transmitted on two different interlaces indifferent symbol periods, as shown in FIG. 7. Techniques for derivingthe coefficients of the time-domain filters for the main and excesschannels are described in commonly assigned U.S. patent application Ser.No. 10/926,884, entitled “Staggered Pilot Transmission for ChannelEstimation and Time Tracking,” filed Aug. 25, 2004.

Different channel estimates may be obtained for the local and wideareas. A wireless device may receive signals from base stations that arefarther away for the wide-area transmission than for the localtransmission. Consequently, the delay spread for the wide-areatransmission may be longer than the delay spread for the localtransmission. A longer channel impulse response estimate (e.g., oflength 3P) may be derived for the wide area. A shorter channel impulseresponse estimate (e.g., of length 2P) may be derived for the localarea.

A longer impulse response estimate for the wide area may be obtained byusing more interlaces for the FDM pilot for the wide area.Alternatively, the same number of interlaces may be used for the FDMpilots for both the local and wide areas, and different time-domainfilters may be used for the local and wide areas. The least-squaresimpulse response estimates for the wide area may be filtered with afirst set of one or more time-domain filters to derive a filteredimpulse response estimate with the desired number of taps (e.g., 3Ptaps) for the wide area. The least-squares impulse response estimatesfor the desired local area may be filtered with a second set oftime-domain filters to derive a filtered impulse response estimate withthe desired number of taps (e.g., 2P taps) for the desired local area.

In general, the time-domain filtering for channel estimation may beperformed based on various considerations such as, e.g., the manner inwhich the FDM pilot is transmitted, the number of interlaces used forthe FDM pilot, the desired length (or the number of taps) for thechannel impulse response estimate, interference suppression and so on.Time-domain filtering may be performed differently on the FDM pilots forthe local and wide-areas to obtain different filtered channel responseestimates for the local and wide-areas.

The filtered impulse response estimate for a given (local or wide) areamay be post-processed to further improve performance. Thepost-processing may include, e.g., setting the last Z taps to zero,where Z may be any integer value, setting taps with energy below apredetermined threshold to zero (thresholding), and so on. Thepost-processed channel taps may be transformed with an DFT to obtain thefinal frequency response estimate used for data detection and decoding.

Referring back to FIG. 5, the transition pilot may be used for channelestimation, time synchronization, acquisition (e.g., automatic gaincontrol (AGC)), and so on. For example, the transition pilot may includethe FDM pilot so that the time-domain filtering for each symbol periodcan be performed on received pilot symbols obtained for the currentsymbol period, at least one earlier symbol period, and at least onelater symbol period. The transition pilot may also be used to obtainimproved timing for the local transmission as well as the wide-areatransmission.

4. Broadcast Transmission and Reception

FIG. 8 shows a flow diagram of a process 800 for broadcasting local andwide-area transmissions in network 100. Each base station in the networkmay perform process 800 in each scheduling interval, which may be, e.g.,each symbol period for FDM structure 300 in FIG. 3A, each frame for TDMstructure 400 in FIG. 4A, or each super-frame for super-frame structure500 in FIG. 5.

Data for a wide-area transmission is processed in accordance with afirst coding and modulation scheme (or mode) selected for the wide-areatransmission to generate data symbols for the wide-area transmission(block 812). Data for a local transmission is processed in accordancewith a second coding and modulation scheme selected for the localtransmission to generate data symbols for the local transmission (block814). Different coding and modulation schemes may be used for the localand wide-area transmissions to achieve the desired coverage. Overheadinformation for the local and wide-area transmissions is determined(blocks 816 and 818). FDM pilot for the wide area, FDM pilot for thelocal area, and transition pilot are generated (blocks, 822, 824, and826, respectively).

The overhead information for the wide-area transmission and the overheadinformation for the local transmission are multiplexed onto theirdesignated transmission spans (blocks 832 and 834). The data symbols forthe wide-area transmission are multiplexed onto a transmission spanallocated for the wide-area transmission (block 836), and pilot symbolsfor the wide-area FDM pilot are multiplexed onto a transmission spanallocated for this pilot (block 838). Similarly, the data symbols forthe local transmission are multiplexed onto a transmission spanallocated for the local transmission (block 840), and pilot symbols forthe local FDM pilot are multiplexed onto a transmission span allocatedfor this pilot (block 842). Each transmission span may correspond to agroup of subbands (e.g., for FDM structure 300), a time segment (e.g.,for TDM structure 400), a group of subbands in a time segment (e.g., forsuper-frame structure 500), or some other time-frequency allocation. TDMand transition pilots, other signaling, and other data may also bemultiplexed (block 844). The multiplexed overhead information, pilots,and data for the local and wide-area transmissions are then broadcast(block 846).

FIG. 9 shows a flow diagram of a process 900 for receiving local andwide-area transmissions broadcast by network 100. A wireless device inthe network may perform process 900 in each scheduling interval.

The wireless device receives a broadcast transmission with both localand wide-area transmissions (block 912). The wireless device processesthe TDM pilot to obtain frame and symbol timing, estimate and correctfrequency error, and so on (block 914). The wireless device identifieswide-area and local channels being served using the WIC and LIC,respectively, which are shown in FIG. 5 (block 916). The wireless devicemay thereafter recover the local transmission, the wide-areatransmission, or both the local and wide-area transmissions from thereceived broadcast transmission.

If the wireless device is receiving the wide-area transmission, asdetermined in block 920, then the wireless device demultiplexes andprocesses overhead information for the wide-area transmission todetermine the time-frequency location of each wide-area channel ofinterest (block 922). The wireless device also demultiplexes andprocesses the wide-area FDM and transition pilots from the transmissionspans allocated for these pilots (block 924) and derives a channelestimate for the wide area (block 926). The wireless devicedemultiplexes data symbols for the wide-area channels of interest fromthe transmission span allocated for the wide-area transmission (block928). The wireless device then processes the data symbols for thewide-area transmission with the wide-area channel estimate and furtherin accordance with a demodulation and decoding scheme applicable for thewide-area transmission and recovers the data for each wide-area channelof interest (block 930).

If the wireless device is receiving the local transmission, asdetermined in block 940, then the wireless device demultiplexes andprocesses overhead information for the local transmission to determinethe time-frequency location of each local channel of interest (block942). The wireless device also demultiplexes and processes the local FDMand transition pilots from the transmission spans allocated for thesepilots (block 944) and derives a channel estimate for the desired localarea (block 946). The wireless device demultiplexes data symbols for thelocal channels of interest from the transmission span allocated for thelocal transmission (block 948). The wireless device then processes thedata symbols for the local transmission with the local channel estimateand further in accordance with a demodulation and decoding schemeapplicable for the local transmission and recovers the data for eachlocal channel of interest (block 950).

If the wireless device is receiving both local and wide-areatransmissions, then the wireless device may perform the processing in adifferent order than the order shown in FIG. 9. For example, thewireless device may demultiplex and process the overhead information forboth local and wide-area transmissions as this information is received.

5. System

FIG. 10 shows a block diagram of a base station 1010 and a wirelessdevice 1050 in wireless broadcast network 100 in FIG. 1. Base station1010 is generally a fixed station and may also be called an accesspoint, a transmitter, or some other terminology. Wireless device 1050may be fixed or mobile and may also be called a user terminal, a mobilestation, a receiver, or some other terminology. Wireless device 1050 mayalso be a portable unit such as a cellular phone, a handheld device, awireless module, a personal digital assistant (PDA), and so on.

At base station 1010, a transmit (TX) data processor 1022 receives datafor a wide-area transmission from sources 1012, processes (e.g.,encodes, interleaves, and symbol maps) the wide-area data, and generatesdata symbols for the wide-area transmission. A data symbol is amodulation symbol for data, and a modulation symbol is a complex valuefor a point in a signal constellation for a modulation scheme (e.g.,M-PSK, M-QAM, and so on). TX data processor 1022 also generates the FDMand transition pilots for the wide area in which base station 1010belongs and provides the data and pilot symbols for the wide area to amultiplexer (Mux) 1026. A TX data processor 1024 receives data for alocal transmission from sources 1014, processes the local data, andgenerates data symbols for the local transmission. TX data processor1024 also generates the FDM and transition pilots for the local area inwhich base station 1010 belongs and provides the data and pilot symbolsfor the local area to multiplexer 1026. The coding and modulation fordata may be selected based on various factors such as, for example,whether the data is for wide-area or local transmission, the data type,the desired coverage for the data, and so on.

Multiplexer 1026 multiplexes the data and pilot symbols for the localand wide areas as well as symbols for overhead information and the TDMpilot onto the subbands and symbol periods allocated for these symbols.A modulator (Mod) 1028 performs modulation in accordance with themodulation technique used by network 100. For example, modulator 1028may perform OFDM modulation on the multiplexed symbols to generate OFDMsymbols. A transmitter unit (TMTR) 1032 converts the symbols frommodulator 1028 into one or more analog signals and further conditions(e.g., amplifies, filters, and frequency upconverts) the analogsignal(s) to generate a modulated signal. Base station 1010 thentransmits the modulated signal via an antenna 1034 to wireless devicesin the network.

At wireless device 1050, the transmitted signal from base station 1010is received by an antenna 1052 and provided to a receiver unit (RCVR)1054. Receiver unit 1054 conditions (e.g., filters, amplifies, andfrequency downconverts) the received signal and digitizes theconditioned signal to generate a stream of data samples. A demodulator(Demod) 1060 performs (e.g., OFDM) demodulation on the data samples andprovides received pilot symbols to a synchronization (Sync)/channelestimation unit 1080. Unit 1080 also receives the data samples fromreceiver unit 1054, determines frame and symbol timing based on the datasamples, and derives channel estimates for the local and wide areasbased on the received pilot symbols for these areas. Unit 1080 providesthe symbol timing and channel estimates to demodulator 1060 and providesthe frame timing to demodulator 1060 and/or a controller 1090.Demodulator 1060 performs data detection on the received data symbolsfor the local transmission with the local channel estimate, performsdata detection on the received data symbols for the wide-areatransmission with the wide-area channel estimate, and provides detecteddata symbols for the local and wide-area transmissions to ademultiplexer (Demux) 1062. The detected data symbols are estimates ofthe data symbols sent by base station 1010 and may be provided inlog-likelihood ratios (LLRs) or some other form.

Demultiplexer 1062 provides detected data symbols for all wide-areachannels of interest to a receive (RX) data processor 1072 and providesdetected data symbols for all local channels of interest to an RX dataprocessor 1074. RX data processor 1072 processes (e.g., deinterleavesand decodes) the detected data symbols for the wide-area transmission inaccordance with an applicable demodulation and decoding scheme andprovides decoded data for the wide-area transmission. RX data processor1074 processes the detected data symbols for the local transmission inaccordance with an applicable demodulation and decoding scheme andprovides decoded data for the local transmission. In general, theprocessing by demodulator 1060, demultiplexer 1062, and RX dataprocessors 1072 and 1074 at wireless device 1050 is complementary to theprocessing by modulator 1028, multiplexer 1026, and TX data processors1022 and 1024, respectively, at base station 1010.

Controllers 1040 and 1090 direct operation at base station 1010 andwireless device 1050, respectively. Memory units 1042 and 1092 storeprogram codes and data used by controllers 1040 and 1090, respectively.A scheduler 1044 schedules the broadcast of local and wide-areatransmissions and allocates and assigns resources for the differenttransmission types.

For clarity, FIG. 10 shows the data processing for the local andwide-area transmissions being performed by two different data processorsat both base station 1010 and wireless device 1050. The data processingfor all types of transmission may be performed by a single dataprocessor at each of base station 1010 and wireless device 1050. FIG. 10also shows the processing for two different types of transmission. Ingeneral, any number of types of transmission with different coverageareas may be transmitted by base station 1010 and received by wirelessdevice 1050. For clarity, FIG. 10 also shows all of the units for basestation 1010 being located at the same site. In general, these units maybe located at the same or different sites and may communicate viavarious communication links. For example, data sources 1012 and 1014 maybe located off site, transmitter unit 1032 and/or antenna 1034 may belocated at a transmit site, and so on.

The multiplexing schemes described herein (e.g., in FIGS. 3A, 4A and 5)have various advantages over a conventional scheme that broadcastsdifferent types of transmission on different RF channels. First, themultiplexing schemes described herein can provide more frequencydiversity than the conventional scheme since each type of transmissionis transmitted across the entire system bandwidth instead of on a singleRF channel. Second, the multiplexing schemes described herein allowreceiver unit 1054 to receive and demodulate all types of transmissionwith a single RF unit that is tuned to a single RF frequency. Thissimplifies the design of the wireless device. In contrast, theconventional scheme may require multiple RF units to recover thedifferent types of transmission sent on different RF channels.

The techniques described herein for broadcasting different types oftransmission over the air may be implemented by various means. Forexample, these techniques may be implemented in hardware, software, or acombination thereof. For a hardware implementation, the processing unitsat a base station used to broadcast different types of transmission maybe implemented within one or more application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), fieldprogrammable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described herein, or a combination thereof. Theprocessing units at a wireless device used to receive different types oftransmission may also be implemented within one or more ASICs, DSPs, andso on.

For a software implementation, the techniques described herein may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The software codes may be storedin a memory unit (e.g., memory unit 1042 or 1092 in FIG. 10) andexecuted by a processor (e.g., controller 1040 or 1090). The memory unitmay be implemented within the processor or external to the processor, inwhich case it can be communicatively coupled to the processor viavarious means as is known in the art.

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of broadcasting data in a wireless broadcast network,comprising: multiplexing data for a wide-area transmission onto a firsttransmission span, the wide-area transmission being sent from aplurality of transmitters in the network; multiplexing data for a localtransmission onto a second transmission span, wherein the multiplexingis onto frequency subbands pseudo-randomly selected from among allusable frequency subbands and the local transmission being sent from asubset of the plurality of transmitters; selecting the first and secondtransmission spans based on an amount of data to broadcast for thewide-area transmission and an amount of data to broadcast for the localtransmission; and broadcasting the local and wide-area transmissions viaa wireless link, wherein the data for the wide-area transmission and thedata for the local transmission are time division multiplexed (TDM) ateach base station, and wherein the first and second transmission spansare first and second time segments, respectively, of a frame of apredetermined time duration.
 2. The method of claim 1, furthercomprising: processing the data for the wide-area transmission inaccordance with a first coding and modulation scheme, wherein theprocessed data for the wide-area transmission is multiplexed onto thefirst transmission span; and processing the data for the localtransmission in accordance with a second coding and modulation scheme,wherein the processed data for the local transmission is multiplexedonto the second transmission span.
 3. The method of claim 2, wherein thesecond coding and modulation scheme has a lower spectral efficiency thanthe first coding and modulation scheme to extend coverage for the localtransmission.
 4. The method of claim 2, wherein the first coding andmodulation scheme has a lower spectral efficiency than the second codingand modulation scheme.
 5. The method of claim 2, further comprising:multiplexing a first pilot onto a third transmission span, the firstpilot being suitable for deriving a first channel estimate for thewide-area transmission; and multiplexing a second pilot onto a fourthtransmission span, the second pilot being suitable for deriving a secondchannel estimate for the local transmission.
 6. The method of claim 5,wherein the first coding and modulation scheme has a lower spectralefficiency than the second coding and modulation scheme.
 7. The methodof claim 6, wherein the first and second coding and modulation schemesare selected based on desired coverage for the wide-area and localtransmissions, respectively.
 8. The method of claim 5, furthercomprising generating the second pilot with a pseudo-random sequenceassigned to the subset of the plurality of transmitters, wherein thesecond pilot for the subset of the plurality of transmitters ispseudo-random with respect to at least one other second pilot for atleast one other subset of the plurality of transmitters.
 9. The methodof claim 1, further comprising adjusting the first and secondtransmission spans based on time of day.
 10. The method of claim 9,wherein the first and second transmission spans are based on apredetermined schedule.
 11. An apparatus in a wireless broadcastnetwork, comprising: means for multiplexing data for a wide-areatransmission onto a first transmission span, the wide-area transmissionbeing sent from a plurality of transmitters in the network; means formultiplexing data for a local transmission onto a second transmissionspan, wherein the multiplexing is onto frequency subbandspseudo-randomly selected from among all usable frequency subbands andthe local transmission being sent from a subset of the plurality oftransmitters; means for selecting the first and second transmissionspans based on an amount of data to broadcast for the wide-areatransmission and an amount of data to broadcast for the localtransmission; and means for broadcasting the local and wide-areatransmissions via a wireless link, wherein the wireless broadcastnetwork utilizes orthogonal frequency division multiplexing (OFDM),wherein the data for the wide-area transmission and the data for thelocal transmission are time division multiplexed (TDM) at each basestation, and wherein the first and second transmission spans are firstand second time segments, respectively, of a frame of a predeterminedtime duration.
 12. The apparatus of claim 11, further comprising: meansfor processing the data for the wide-area transmission in accordancewith a first coding and modulation scheme, wherein the processed datafor the wide-area transmission is multiplexed onto the firsttransmission span; and means for processing the data for the localtransmission in accordance with a second coding and modulation scheme,wherein the processed data for the local transmission is multiplexedonto the second transmission span.
 13. The apparatus of claim 12,wherein the second coding and modulation scheme has a lower spectralefficiency than the first coding and modulation scheme to extendcoverage for the local transmission.
 14. The apparatus of claim 12,wherein the first coding and modulation scheme has a lower spectralefficiency than the second coding and modulation scheme.
 15. Theapparatus of claim 12, further comprising: means for multiplexing afirst pilot onto a third transmission span, the first pilot beingsuitable for deriving a first channel estimate for the wide-areatransmission; and means for multiplexing a second pilot onto a fourthtransmission span, the second pilot being suitable for deriving a secondchannel estimate for the local transmission.
 16. The apparatus of claim15, wherein the first coding and modulation scheme has a lower spectralefficiency than the second coding and modulation scheme.
 17. Theapparatus of claim 16, wherein the first and second coding andmodulation schemes are selected based on desired coverage for thewide-area and local transmissions, respectively.
 18. The apparatus ofclaim 15, further comprising means for generating the second pilot witha pseudo-random sequence assigned to the subset of the plurality oftransmitters, wherein the second pilot for the subset of the pluralityof transmitters is pseudo-random with respect to at least one othersecond pilot for at least one other subset of the plurality oftransmitters.
 19. The apparatus of claim 11, further comprising meansfor adjusting the first and second transmission spans based on time ofday.
 20. The apparatus of claim 19, wherein the first and secondtransmission spans are based on a predetermined schedule.
 21. A wirelessdevice for broadcasting data in a wireless broadcast network,comprising: a multiplexer for multiplexing data for a wide-areatransmission onto a first transmission span, the wide-area transmissionbeing sent from a plurality of transmitters in the network, and formultiplexing data for a local transmission onto a second transmissionspan, wherein the multiplexing is onto frequency subbandspseudo-randomly selected from among all usable frequency subbands andthe local transmission being sent from a subset of the plurality oftransmitters; a processor coupled to the multiplexer for selecting thefirst and second transmission spans based on an amount of data tobroadcast for the wide-area transmission and an amount of data tobroadcast for the local transmission; and a transmitter coupled to theprocessor for broadcasting the local and wide-area transmissions via awireless link, wherein the data for the wide-area transmission and thedata for the local transmission are time division multiplexed (TDM) ateach base station, and wherein the first and second transmission spansare first and second time segments, respectively, of a frame of apredetermined time duration.
 22. The wireless device of claim 21,wherein the processor further comprises a wide-area transmit dataprocessor for processing the data for the wide-area transmission inaccordance with a first coding and modulation scheme, wherein theprocessed data for the wide-area transmission is multiplexed onto thefirst transmission span; and a local transmit data processor forprocessing the data for the local transmission in accordance with asecond coding and modulation scheme, wherein the processed data for thelocal transmission is multiplexed onto the second transmission span. 23.The wireless device of claim 22, wherein the second coding andmodulation scheme has a lower spectral efficiency than the first codingand modulation scheme to extend coverage for the local transmission. 24.The wireless device of claim 22, wherein the first coding and modulationscheme has a lower spectral efficiency than the second coding andmodulation scheme.
 25. The wireless device of claim 22, wherein themultiplexer is further configured to multiplex a first pilot onto athird transmission span, the first pilot being suitable for deriving afirst channel estimate for the wide-area transmission, and multiplex asecond pilot onto a fourth transmission span, the second pilot beingsuitable for deriving a second channel estimate for the localtransmission.
 26. The wireless device of claim 25, wherein the firstcoding and modulation scheme has a lower spectral efficiency than thesecond coding and modulation scheme.
 27. The wireless device of claim26, wherein the first and second coding and modulation schemes areselected based on desired coverage for the wide-area and localtransmissions, respectively.
 28. The wireless device of claim 25,wherein the local transmit data processor is further configured togenerate the second pilot with a pseudo-random sequence assigned to thesubset of the plurality of transmitters, wherein the second pilot forthe subset of the plurality of transmitters is pseudo-random withrespect to at least one other second pilot for at least one other subsetof the plurality of transmitters.
 29. The wireless device of claim 21,wherein the processor is further configured to adjust the first andsecond transmission spans based on time of day.
 30. The wireless deviceof claim 29, wherein the first and second transmission spans are basedon a predetermined schedule.
 31. A method of receiving data in awireless broadcast network, comprising: receiving via a wireless link abroadcast transmission comprised of a wide-area transmission and a localtransmission, the wide-area transmission being sent from a plurality oftransmitters in the network, and the local transmission being sent froma subset of the plurality of transmitters; demultiplexing the data forthe wide-area transmission from a first transmission span when thewide-area transmission is received; and demultiplexing the data for thelocal transmission from a second transmission span when the localtransmission is received, wherein the data for the local transmissionare multiplexed onto frequency subbands pseudo-randomly selected fromamong all usable frequency subbands, wherein the data for the wide-areatransmission and the data for the local transmission are time divisionmultiplexed (TDM) at each base station, and wherein the first and secondtransmission spans are a first time segment and a second time segment,respectively, of a frame and are based on an amount of data broadcast onthe wide-area transmission and an amount of data broadcast on the localtransmission.
 32. The method of claim 31, wherein the data for thewide-area transmission is frequency division multiplexed (FDM) with thedata for the local transmission, and wherein the first and secondtransmission spans are obtained with multi-carrier modulation.
 33. Themethod of claim 32, wherein the first time segment for the wide-areatransmission is prior to the second time segment for the localtransmission.
 34. The method of claim 33, wherein the first transmissionspan includes all frequency subbands usable for data transmission in thefirst time segment of the frame, and wherein the second transmissionspan includes all frequency subbands usable for data transmission in thesecond time segment of the frame.
 35. The method of claim 31, furthercomprising: if the wide-area transmission is being received,demultiplexing overhead information for the wide-area transmission froma third transmission span; and if the local transmission is beingreceived, demultiplexing overhead information for the local transmissionfrom a fourth transmission span.
 36. The method of claim 35, wherein theoverhead information for the wide-area transmission indicates frequencyand time location of each data channel for the wide-area transmission,and wherein the overhead information for the local transmissionindicates frequency and time location of each data channel for the localtransmission.
 37. The method of claim 31, further comprising: if thewide-area transmission is being received, demultiplexing a first pilotfrom a third transmission span, deriving a first channel estimate forthe wide-area transmission based on the first pilot, and processing thedata for the wide-area transmission with the first channel estimate; andif the local transmission is being received, demultiplexing a secondpilot from a fourth transmission span, deriving a second channelestimate for the local transmission based on the second pilot, andprocessing the data for the local transmission with the second channelestimate.
 38. The method of claim 37, further comprising: if thewide-area transmission is being received, processing the first pilotwith a first set of at least one time-domain filter to derive the firstchannel estimate; and if the local transmission is being received,processing the second pilot with a second set of at least onetime-domain filter to derive the second channel estimate.
 39. The methodof claim 38, wherein the first and second channel estimates arerespectively associated with a first impulse response estimate and asecond impulse response estimate having different lengths.
 40. Themethod of claim 39, further comprising: performing thresholding to zeroout channel taps of the first impulse response estimate that are below afirst predetermined threshold; and performing thresholding to zero outchannel taps of the second impulse response estimate that are below asecond predetermined threshold.
 41. A wireless device for receiving datain a wireless broadcast network, comprising: a receiver for receivingvia a wireless link a broadcast transmission comprised of a wide-areatransmission and a local transmission, the wide-area transmission beingsent from a plurality of transmitters in the network, and the localtransmission being sent from a subset of the plurality of transmitters;and a demultiplexer configured to demultiplex the data for the wide-areatransmission from a first transmission span if the wide-areatransmission is being received, and to demultiplex the data for thelocal transmission from a second transmission span if the localtransmission is being received, wherein the data for the localtransmission are multiplexed onto frequency subbands pseudo-randomlyselected from among all usable frequency subbands, wherein the data forthe wide-area transmission and the data for the local transmission aretime division multiplexed (TDM) at each base station, and wherein thefirst and second transmission spans are a first time segment and asecond time segment, respectively, of a frame and are based on an amountof data broadcast on the wide-area transmission and an amount of databroadcast on the local transmission.
 42. The wireless device of claim41, wherein the data for the wide-area transmission is frequencydivision multiplexed (FDM) with the data for the local transmission, andwherein the first and second transmission spans are obtained withmulti-carrier modulation.
 43. The wireless device of claim 42, whereinthe first time segment for the wide-area transmission is prior to thesecond time segment for the local transmission.
 44. The wireless deviceof claim 43, wherein the first transmission span includes all frequencysubbands usable for data transmission in the first time segment of theframe, and wherein the second transmission span includes all frequencysubbands usable for data transmission in the second time segment of theframe.
 45. The wireless device of claim 41, wherein the demultiplexer isfurther configured to demultiplex overhead information for the wide-areatransmission from a third transmission span if the wide-areatransmission is being received, and demultiplex overhead information forthe local transmission from a fourth transmission span if the localtransmission is being received.
 46. The wireless device of claim 45,wherein the overhead information for the wide-area transmissionindicates frequency and time location of each data channel for thewide-area transmission, and wherein the overhead information for thelocal transmission indicates frequency and time location of each datachannel for the local transmission.
 47. The wireless device of claim 41,wherein the demultiplexer is further configured to demultiplex a firstpilot from a third transmission span if the wide-area transmission isbeing received and to demultiplex a second pilot from a fourthtransmission span if the local transmission is being received; and thewireless device further comprising: a synchronization/channel estimationunit for deriving a first channel estimate for the wide-areatransmission based on the first pilot if the wide-area transmission isbeing received, and deriving a second channel estimate for the localtransmission based on the second pilot if the local transmission isbeing received; and a demodulator for processing the data for thewide-area transmission with the first channel estimate if the wide-areatransmission is being received, and for processing the data for thelocal transmission with the second channel estimate if the localtransmission is being received.
 48. The wireless device of claim 47,further comprising: a wide-area receive data processor for processingthe first pilot with a first set of at least one time-domain filter toderive the first channel estimate if the wide-area transmission is beingreceived; and a local receive data processor for processing the secondpilot with a second set of at least one time-domain filter to derive thesecond channel estimate if the local transmission is being received. 49.The wireless device of claim 48, wherein the first and second channelestimates are respectively associated with a first impulse responseestimate and a second impulse response estimate having differentlengths.
 50. The wireless device of claim 49, further comprising asynchronization/channel estimation unit for performing thresholding tozero out channel taps of the first impulse response estimate that arebelow a first predetermined threshold, and performing thresholding tozero out channel taps of the second impulse response estimate that arebelow a second predetermined threshold.